1. Field of the Invention
The present invention relates to waste gas flares and, more particularly, to a gas assist assembly for use with such flares.
2. Description of the Prior Art
Flaring is a high temperature oxidation process used to burn combustible components, mostly hydrocarbons, of waste gases from industrial operations. Natural gas, propane, ethylene, propylene, butadiene and butane constitute over 95% of the waste gases flared. Flares are used extensively to dispose of (1) purged and waste products from refineries, (2) unrecoverable gases emerging from oil and gas well instillation, (3) vented gases from blast furnaces, (4) unused gases from coke ovens, and (5) gaseous wastes from chemical industries.
There are generally two types of flares, elevated and ground flares. The present invention is particularly applicable to use with elevated flares. Elevated flares, comprise a flare riser, which can extend from a few feet to several hundred feet above the ground to a flare tip from which the waste gases exit. A waste gas stream is fed through the riser and is combusted at the tip. A typical elevated flare system consists of (1) a gas collection header and piping for collecting gases from processing units, (2) a knockout drum (disentrainment drum) to remove and store condensables, entrained liquids and particulates, (3) a single- or multiple-burner unit, (4) a flare stack riser, and (5) an igniter e.g., a gas pilot or an electronic igniter, to ignite the mixture of waste gas and air, and, if required, (6) a provision for external momentum force e.g., a motive gas such as e.g., steam injection, forced air, or some other gas for smokeless flaring.
Due to process and/or regulatory considerations, various other gases are sometimes added to the released waste gas stream. Examples of other gases that are added to the released gas stream include purge gas (for example, natural gas or nitrogen) and enrichment fuel gas (for example natural gas or propane). The gas stream that arrives at the inlet of the flare tip is referred to as “vent gas” regardless of whether it consists of only the released waste gas or the released waste gas together with other gases e.g., purge gas, motive gas, etc. that have been added thereto. Typically, the vent gas together with all other gases and vapors present in the atmosphere immediately downstream of the flare tip, not including air, but including steam or other assist gas added at the flare tip and fuel gas discharged from the pilot or pilots of the flare assembly, is referred to as “flare gas”.
Purge gas is often added to the released waste gas stream (or otherwise to the flare assembly if a waste gas stream is not being released by the facility at the time) in order to maintain a positive gas flow through the flare assembly and prevent air and possibly other gases from back flowing therein.
Most gas flares are required to operate in a relatively smokeless manner. This is generally achieved by making sure that the vent gas is admixed with a sufficient amount of air in a relatively short period of time to sufficiently oxidize the soot particles formed in the flame. In applications where the gas pressure is low, the momentum of the vent gas stream alone may not be sufficient to provide smokeless operation. In such applications, it is necessary to add an assist medium (gas) to achieve smokeless operation. The assist medium can be used to provide the necessary motive force to entrain ambient air from around the flare apparatus. Examples of useful assist media include steam, air, natural gas, propane, etc. Many factors, including local energy costs and availability, must be taken into account in selecting a smoke suppressing medium (gas assist).
A common assist medium for adding momentum to low-pressure gases is air which is typically injected through one or more groups of nozzles that are associated with i.e., adjacent the flare tip. In using gas assist, the assist assembly has jets which eject the assist gas into the discharge vent gas with high levels of turbulence.
For cost savings it is desirable to have a flare gas assist assembly which minimizes the amount of gas employed to generate the assist gas while at the same time enhancing the formation of a smokeless flare.